Various control schemes have been devised to regulate large capacity compressors, particularly those having adjustable guide vanes. An early showing of such an arrangement, with an associated electronic control system, is described and claimed in U.S. Pat. No. 3,355,906--Newton, entitled "Refrigeration System Including Control For Varying Compressor Speed," which issued Dec. 5, 1967 and is assigned to the assignee of this invention. The method of control there disclosed included providing signals related to the discharge pressure and to the suction pressure of the compressor, and deriving a control signal related to the ratio of these pressures. This signal was then used in regulating the speed of the electrical motor driving the compressor. Later considerable work was directed to the refinement of the control arrangement, particularly the interaction between the compressor's inlet guide vanes and the speed regulation of the electrical motor driving the compressor. A significant step forward in this art using the improved control system is described and claimed in U.S. Pat. No. 4,151,725--Kountz, Erth and Bauman, entitled "Control System for Regulating Large Capacity Rotating Machinery," which issued May 1, 1979 to the assignee of this invention. In the disclosed arrangement the compressor head value was inferred from the difference in two temperatures, one of which is related to the saturated refrigerant vapor leaving the evaporator, and the other the refrigerant in the condenser discharge line. Even with this significant step forward, operation of the system there described did not produce as great a saving at light loads and at low heads as was desired. Considerable analysis was directed to the problem, and it appears at this time that the assumed linearity of the function between the isentropic compressor head parameter, shown as .OMEGA., and the difference between the condensing and evaporating refrigerant temperatures was not applicable over system operation at light loads and at low heads. In addition there are thermal lags involved in measuring the compressor head through inference, by utilizing refrigerant temperatures and taking the difference as described, as contrasted the determination of the head value from refrigerant pressures.
It is therefore a primary object of the present invention to provide an improved control system for such large capacity, rotating machinery which is more effective than the known systems, including those in the referenced patents, and especially shows improvement in system operation at light loads and at low heads.
It is a more specific object of the present invention to derive a new control signal, from quantities readily measurable in existing systems, which will more precisely identify the compressor head value at light loads and low heads, and thus lead to improved operation and energy conservation under these conditions.